Please refer to FIGS. 7A˜7D, which are the cross-sectional schematics showing steps of a fabricating method for bulk acoustic wave filter of conventional technology. In FIG. 7A, a recess 74 and a recess 74′ are formed by etching a top surface of a silicon substrate 75. Forming a sacrificial layer 77 on the silicon substrate 75 and then polishing the sacrificial layer 77 by a chemical-mechanical planarization (CMP) process such that the sacrificial layer 77 above the top surface of the silicon substrate 75 are removed and the structure shown as in FIG. 7B are formed, wherein the recess 74 and the recess 74′ are filled up with the sacrificial layer 77. In FIG. 7C, a first bulk acoustic wave resonance structure 70 and a second bulk acoustic wave resonance structure 70′ are formed on the top surface of the silicon substrate 75, wherein the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′ have a same thickness bottom electrode 71 and a same thickness piezoelectric layer 72 respectively, wherein the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′ have a top electrode 73 and a top electrode 73′ respectively, wherein a thickness of the top electrode 73 is unequal to a thickness of the top electrode 73′. The top electrode 73 and the top electrode 73′ have a thickness difference 76. In FIG. 7D, the sacrificial layer 77 filled up in the recess 74 and the recess 74′ are etched such that the recess 74 and the recess 74′ form two cavities of the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′ respectively. Since the thickness of the top electrode 73′ is thicker than the thickness of the top electrode 73 such that a resonance frequency of the second bulk acoustic wave resonance structure 70′ is lower than a resonance frequency of the first bulk acoustic wave resonance structure 70. The first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′ have a resonance frequency difference, wherein the resonance frequency difference is correlated to the thickness difference 76.
However, tuning the resonance frequency difference of the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′ by adjusting the thickness difference 76 of the top electrode 73 and the top electrode 73′ is essentially achieving tuning the resonance frequency difference by the structure difference between the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′. It increases the complexity for fabricating the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′, furthermore it may affect the characteristics performance of the first bulk acoustic wave resonance structure 70 and the second bulk acoustic wave resonance structure 70′.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.